1 //=== UndefResultChecker.cpp ------------------------------------*- C++ -*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This defines UndefResultChecker, a builtin check in ExprEngine that
11 // performs checks for undefined results of non-assignment binary operators.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "ClangSACheckers.h"
16 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
17 #include "clang/StaticAnalyzer/Core/Checker.h"
18 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
19 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
20 #include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
21 #include "llvm/ADT/SmallString.h"
22 #include "llvm/Support/raw_ostream.h"
23 
24 using namespace clang;
25 using namespace ento;
26 
27 namespace {
28 class UndefResultChecker
29   : public Checker< check::PostStmt<BinaryOperator> > {
30 
31   mutable std::unique_ptr<BugType> BT;
32 
33 public:
34   void checkPostStmt(const BinaryOperator *B, CheckerContext &C) const;
35 };
36 } // end anonymous namespace
37 
38 static bool isArrayIndexOutOfBounds(CheckerContext &C, const Expr *Ex) {
39   ProgramStateRef state = C.getState();
40   const LocationContext *LCtx = C.getLocationContext();
41 
42   if (!isa<ArraySubscriptExpr>(Ex))
43     return false;
44 
45   SVal Loc = state->getSVal(Ex, LCtx);
46   if (!Loc.isValid())
47     return false;
48 
49   const MemRegion *MR = Loc.castAs<loc::MemRegionVal>().getRegion();
50   const ElementRegion *ER = dyn_cast<ElementRegion>(MR);
51   if (!ER)
52     return false;
53 
54   DefinedOrUnknownSVal Idx = ER->getIndex().castAs<DefinedOrUnknownSVal>();
55   DefinedOrUnknownSVal NumElements = C.getStoreManager().getSizeInElements(
56       state, ER->getSuperRegion(), ER->getValueType());
57   ProgramStateRef StInBound = state->assumeInBound(Idx, NumElements, true);
58   ProgramStateRef StOutBound = state->assumeInBound(Idx, NumElements, false);
59   return StOutBound && !StInBound;
60 }
61 
62 static bool isShiftOverflow(const BinaryOperator *B, CheckerContext &C) {
63   return C.isGreaterOrEqual(
64       B->getRHS(), C.getASTContext().getIntWidth(B->getLHS()->getType()));
65 }
66 
67 void UndefResultChecker::checkPostStmt(const BinaryOperator *B,
68                                        CheckerContext &C) const {
69   ProgramStateRef state = C.getState();
70   const LocationContext *LCtx = C.getLocationContext();
71   if (state->getSVal(B, LCtx).isUndef()) {
72 
73     // Do not report assignments of uninitialized values inside swap functions.
74     // This should allow to swap partially uninitialized structs
75     // (radar://14129997)
76     if (const FunctionDecl *EnclosingFunctionDecl =
77         dyn_cast<FunctionDecl>(C.getStackFrame()->getDecl()))
78       if (C.getCalleeName(EnclosingFunctionDecl) == "swap")
79         return;
80 
81     // Generate an error node.
82     ExplodedNode *N = C.generateErrorNode();
83     if (!N)
84       return;
85 
86     if (!BT)
87       BT.reset(
88           new BuiltinBug(this, "Result of operation is garbage or undefined"));
89 
90     SmallString<256> sbuf;
91     llvm::raw_svector_ostream OS(sbuf);
92     const Expr *Ex = nullptr;
93     bool isLeft = true;
94 
95     if (state->getSVal(B->getLHS(), LCtx).isUndef()) {
96       Ex = B->getLHS()->IgnoreParenCasts();
97       isLeft = true;
98     }
99     else if (state->getSVal(B->getRHS(), LCtx).isUndef()) {
100       Ex = B->getRHS()->IgnoreParenCasts();
101       isLeft = false;
102     }
103 
104     if (Ex) {
105       OS << "The " << (isLeft ? "left" : "right") << " operand of '"
106          << BinaryOperator::getOpcodeStr(B->getOpcode())
107          << "' is a garbage value";
108       if (isArrayIndexOutOfBounds(C, Ex))
109         OS << " due to array index out of bounds";
110     } else {
111       // Neither operand was undefined, but the result is undefined.
112       if ((B->getOpcode() == BinaryOperatorKind::BO_Shl ||
113            B->getOpcode() == BinaryOperatorKind::BO_Shr) &&
114           C.isNegative(B->getRHS())) {
115         OS << "The result of the "
116            << ((B->getOpcode() == BinaryOperatorKind::BO_Shl) ? "left"
117                                                               : "right")
118            << " shift is undefined because the right operand is negative";
119       } else if ((B->getOpcode() == BinaryOperatorKind::BO_Shl ||
120                   B->getOpcode() == BinaryOperatorKind::BO_Shr) &&
121                  isShiftOverflow(B, C)) {
122 
123         OS << "The result of the "
124            << ((B->getOpcode() == BinaryOperatorKind::BO_Shl) ? "left"
125                                                               : "right")
126            << " shift is undefined due to shifting by ";
127 
128         SValBuilder &SB = C.getSValBuilder();
129         const llvm::APSInt *I =
130             SB.getKnownValue(C.getState(), C.getSVal(B->getRHS()));
131         if (!I)
132           OS << "a value that is";
133         else if (I->isUnsigned())
134           OS << '\'' << I->getZExtValue() << "\', which is";
135         else
136           OS << '\'' << I->getSExtValue() << "\', which is";
137 
138         OS << " greater or equal to the width of type '"
139            << B->getLHS()->getType().getAsString() << "'.";
140       } else if (B->getOpcode() == BinaryOperatorKind::BO_Shl &&
141                  C.isNegative(B->getLHS())) {
142         OS << "The result of the left shift is undefined because the left "
143               "operand is negative";
144       } else {
145         OS << "The result of the '"
146            << BinaryOperator::getOpcodeStr(B->getOpcode())
147            << "' expression is undefined";
148       }
149     }
150     auto report = llvm::make_unique<BugReport>(*BT, OS.str(), N);
151     if (Ex) {
152       report->addRange(Ex->getSourceRange());
153       bugreporter::trackNullOrUndefValue(N, Ex, *report);
154     }
155     else
156       bugreporter::trackNullOrUndefValue(N, B, *report);
157 
158     C.emitReport(std::move(report));
159   }
160 }
161 
162 void ento::registerUndefResultChecker(CheckerManager &mgr) {
163   mgr.registerChecker<UndefResultChecker>();
164 }
165